Observing responses maintained by re-presentations of S-

Food-deprived pigeons were given a random sequence of 50-s discrete trials, half of which ended with and half without response-independent food presentation. During a trial, pecking a white key could change its color to red or green, depending on whether food (S+ color) or no food (S- color) was scheduled. In the single stimulus-production condition (SSP), if the red or the green key color was produced, the key remained red or green, respectively, until the end of the trial. In the multiple stimulus-production condition (MSP), the duration of each green or red illumination was limited to a maximum of 10 s. After 10 s, the white light resumed and additional key-color changes (re-presentations of S+ or S-) were possible. Results showed that S+ and S- were produced equally often in SSP, as well as in the first stimulus production in MSP. The frequencies of S- re-presentations, however, were clearly and systematically higher than the frequencies of S+ re-presentations. These findings demonstrate that under certain conditions, a stimulus signaling the absence of primary reinforcement (S-) can maintain higher rates of observing behavior than a stimulus signaling its presence (S+).